As observers move through the environment they experience radial optic flow as retinal images stream away from a central focus of expansion. Such optic flow is a rich source of information about both the 3-dimensional layout of our environment and the observer's movements within it, and has been the subject of intense psychophysical, physiological and computational studies in recent years. An aspect of this situation that has been overlooked is that, as the observer moves forwards he/she must be constantly adjusting the vergence angle between his/her two eyes in order to maintain binocular alignment on the objects of interest that lie ahead. The primary visual cue used to control this convergence angle is binocular disparity, though size changes as well as static perspective and overlay cues can also play a role. However, it occurred to us that, to the extent that it charts the observer's rate of forward progress, optic flow itself might also make a contribution. Subjects faced a tangent screen onto which large random-dot patterns were back projected. Steps of radial flow - achieved by switching between 2 projected images - elicited pure horizontal vergence (i.e., negligible vertical vergence) at ultra-short latencies (80-100 ms) in all 4 human subjects tested. As expected from the optical geometry, expanding or centrifugal flow (signalling forward approach) resulted in increased convergence whereas contracting or centripetal flow (signalling backward retreat) resulted in decreased convergence. We also showed that such binocular vergence was elicited even when one eye was covered and the other saw only half the radial flow pattern, consistent with the existence of a neural decoding mechanism selectively sensitive to radial flow. Such responses share a number of properties with other visual tracking mechanisms that assist the moving observer to stabilize his/her eyes on objects off to one side and we hypothesize that flow-induced vergence is but one of a family of rapid ocular reflexes that are mediated by the medial superior temporal (MST) area of cortex and compensate for translational disturbances of the observer.